1. Field of the Invention
The invention concerns filters that remove particulates from streams of hot gases such as from exhausts of fossil fuel furnaces, coal-gasification plants, and internal combustion engines. The invention is especially concerned with the need to remove particulate matter from exhausts of diesel engines.
2. Description of the Related Art
As part of a worldwide effort to protect the environment, concerted efforts are underway to reduce the quantity of soot and other particulate matter emitted into the atmosphere by the burning of fossil fuels. For example, particulate emissions from coal-burning furnaces, e.g., of electric power plants, have long been a primary cause of environmental concern. Efforts also are being made to reduce the emission of soot from the exhausts of diesel engines and coal-gasification plants.
It is known that soot can be removed from hot exhaust gases by passing such gases through filter fabrics. One such filter fabric, as disclosed in coassigned U.S. Pat. No. 4,543,113 (Forester et al.), is made from a woven ceramic fiber which maintains its integrity for prolonged periods at temperatures up to at least 1150.degree. C. and for at least short periods up to at least 1400.degree. C. This filter fabric preferably is employed as a seamless, tubular filter which can be cleaned periodically by a reverse purge of air. Also illustrated is a flat filter having a flat piece of the fabric supported in a frame. Preferably, the ceramic fiber fabric comprises continuous alumina-boria-silica fibers (such as are sold under the trademark NEXTEL.TM. 312 from the 3M Co.).
Although little used commercially to date, a variety of diesel particulate traps have been described in the prior art. Most employ a plurality of rigid, perforated hollow tubes on which ceramic yarn is wound. In Offenlegungsschrift No. DE 37 31 766 (Buck), laid open 30 Mar. 1989, perforated tubes are covered with a knitted sleeve of ceramic yarn of alumina-boria-silica fibers or wound with a length of the knitted fabric or, as shown in FIG. 11, a spirally wound length of the knitted fabric is stuffed into a canister.
In some diesel particulate traps, the perforated hollow tubes extend side-by-side, and in others, the tubes are concentric. Side-by-side, rigid, perforated hollow tubes (called "support pipes") are shown in U.S. Pat. No. 4,324,572 (Erdmannsdoerfer). There, threads of spun silicon dioxide or silica fiber are wound on each of the tubes to provide filtering elements. Each tube is blocked at the exhaust inlet, while channels between the tubes are blocked by a wall at the outlet, thus forcing the exhaust to pass through the filtering elements.
Hardenberg et al., "Experiences in the Development of Ceramic Fiber Coil Particulate Traps," SAE Technical Paper Series, International Congress and Exposition, Detroit, Mich., Feb. 23-27, 1987, pp 67-70, reports testing of a ceramic fiber coil trap made using threads of endless ceramic fibers which are twisted together. The threads are wound crosswise onto perforated hollow tubes to provide filtering elements. Unfortunately, the crosswinding process is tedious and slow, so that such a diesel particulate trap may be too expensive for widespread commercial use.
Any filter that removes particulate matter from hot exhaust gases should not substantially increase back pressures and thus should not substantially interfere with filtration efficiency. Nevertheless, known diesel particulate traps tend to increase back pressures to such an extent that they unduly increase fuel consumption. They also tend to be inefficient and to have short useful lives, and if regeneratable, would require undesirable frequent regeneration cycles.